Nickel-based catalyst systems including a nickel-containing compound, an organoaluminum compound, and a fluorine-containing compound may be employed for polymerizing conjugated dienes into cis 1,4-polydienes, for example, polymerizing 1,3-butadiene to form cis-1,4-polybutadiene. The cis-1,4-polydienes produced by nickel-based catalyst systems can have a broad molecular weight distribution. Because a polymer having a narrower molecular weight distribution may offer certain advantages such as higher abrasion resistance, lower hysteresis, and better tensile properties, it is desirable to develop a nickel-based catalyst system that is capable of producing cis-1,4-polydienes having a narrower molecular weight distribution.